CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

Theoretical consideration of the growth kinetics for GaAs and GaSb

An extended MOMBE growth kinetics model is proposed, based on the Robertson model, to explain both the GaAs growth rate variation and modulated beam mass spectroscopy data reported by Martin and Whitehouse. In this model, we assume that (1) MEGa molecules react with ethyl-radicals to form DEGa, (2) excessive group-V molecules on the surface suppress the decomposition of DEGa and enhance the desorption of DEGa, (3) reaction of DEGa with ethyl-radicals to form TEGa is negligible, and (4) effective surface coverage of excessive group-V atoms during growth is determined by the double layer adsorption model including desorption parameters for group-V molecules. The first assumption (1) is found to be a dominant process to explain the behaviour of DEGa desorption at high temperatures. This model can reproduce the dependences of both growth rate and desorbing rate of Ga alkyls on substrate temperature during GaAs MOMBE growth. The use of Sb instead of As produces a significant change in the growth rate variation with substrate temperature and group-V flux for the growth of GaSb, in spite of the use of the same TEGa flow rate. This can be rationalized by the difference in the desorption parameters for Sb and As.     

Applications of MBMS and surface spectroscopic techniques in the study of reaction mechanisms in CBE; investigations of the reactivity of tritertiarybutylgallium and triisobutylgallium as alternative precursors for epilayer growth

Current approaches to the study of reaction mechanisms in CBE and investigations of the potential of triisobutylgallium and tritertiarybutylgallium as novel CBE precursors are reviewed. Surface spectroscopic techniques indicate that adsorbed iso-butyl radicals decompose to produce gaseous butene and hydrogen at significantly lower temperatures than in the corresponding process for ethyl radicals on GaAs, resulting in lowered growth temperatures and low temperature C incorporation levels in comparison to the results obtained with triethylgallium. A β-methyl migration occurs at higher temperatures causing C to deposit irreversibly on the surface in the presence of Al. Lowered temperatures for the β-hydride elimination reaction are also observed for adsorbed tertiarybutyl radicals and the absence of β-methyl groups avoids the facile C deposition process seen for iso-butyl. These potential advantages associated with tertiarybutyl ligands cannot be realized straightforwardly in CBE using tritertiarybutylgallium however, since steric crowding effects result in the inefficient total dissociation of the adsorbing precursor molecule.     

CBE growth of GaAs/GaAlAs HBTs using the new DEAlH-NMe3 precursor and all-gaseous dopants

This paper describes the first reported use of diethylaluminium hydride-trimethylamine adduct (DEAlH-NMe₃) for the growth of GaAs/GaAlAs power heterojunction bipolar transistors (HBTs) by chemical beam epitaxy (CBE). This precursor possesses a significantly higher vapour pressure than the more conventionally used triethylaluminium (TEA), and leads to much less stringent requirements for bubbler and gas-line heating, and also much-improved GaAs/GaAlAs heterojunction definition when no carrier gas is employed. The use of all-gaseous n- and p-type dopants offers significant technological advantages in CBE, and the current paper also provides the first report of the use of hydrogen sulphide for n-type doping of CBE-grown GaAlAs HBT emitter regions. In conclusion, DC and RF data obtained from the heterojunction bipolar transistors fabricated to date are described. A DC gain of 40 has already been measured and encouraging early data obtained from RF-probed devices are also presented.     

Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

Thermodynamic analysis of GaAs growth by molecular beam epitaxy at the surface structure transition from 3 × 1 to 4 × 2

The molecular beam epitaxy (MBE) growth of GaAs layers on a single crystal is studied in relation with the stability domain of the GaAs compound in the Ga-As binary system. The growth parameters, i.e. The Ga and As impinging atomic flows, are compared to the necessary flows as calculated by thermodynamics. In order to take into account in the real growth situation, which is not strictly at equilibrium, the flow balance at the surface of the crystal between the impinging flows and the growth and evaporated flows is written for quasi-equilibrium growth conditions, including condensation and evaporation coefficients that split the “so-called” sticking coefficient in parts related to the condensation or the evaporation phenomenon for each gaseous species. A comparison between the quasi-equilibrium simulation of the growth and the experiment is made with the assumption that the surface structure transition from gallium-stabilized to arsenic-stabilized surface corresponds to the growth of a GaAs crystal at its solidus boundary rich in gallium. The surface structure transitions are observed by reflection high energy electron diffraction (RHEED) and the impinging atomic flows are carefully calibrated and also controlled by RHEED oscillations as observed after gallium or arsenic excess as deposited on the surface. The results show that the growth is effectively performed close to equilibrium conditions as evidenced by the values of the condensations and evaporation coefficients. The evaporation coefficient of gallium is 0.4, showing that this component is the supersaturated one at the surface, and this value agrees with theoretical predictions for an evaporation process (during growth) controlled by the surface diffusion process of monoatomic species between steps.     

Effect of surface kinetics on the dendritic growth of ice in supercooled water

The dendritic growth of ice in supercooled water was investigated experimentally. It has been found that, with an increase in the degree of initial supercooling of water, the data of measuring the growth rate of the dendrite tip v _t, the average distance between the tip and the first side branch z? _SB, and the fractal dimension d _f of the dendrite contour diverge from the analytically and numerically calculated parameters of dendritic growth. It is shown that the discrepancy between experiment and theory is due to the transition from the diffusional growth to the growth controlled by the surface kinetics. The nature of the anisotropic surface kinetics of ice growth in supercooled water is discussed.     

Structure of porous surface layers of single-crystal GaAs(001) wafers from data of X-ray diffractometry and reflectometry

The surface morphology and structure parameters of the surface layers of the single-crystal GaAs(001) wafers subjected to He⁺ ion implantation (E = 300 keV, D = 10¹⁶ atoms/cm²) and subsequent electrochemical etching in a 0.5 M H₂SO₄ aqueous solution are investigated by X-ray reflectometry and double-crystal X-ray diffractometry. The strain and amorphization profiles over the layer thickness are determined from X-ray diffraction data. The density of surface layers, their thickness, and the changes of the surface relief upon etching are evaluated from the reflectometric data. The experimental parameters of the studied layers are compared with the theoretical distributions of implanted impurities and intrinsic point defects, which are calculated by the Monte Carlo method. A correlation is revealed between the layer thickness and the depth of the maximum defect concentration. It is found that the electrochemical etching predominantly occurs in strongly amorphized regions of the surface layer and does not lead to a change in the total layer thickness. The results of X-ray diffraction investigations are confirmed by scanning electron microscopy.     

Optical second harmonic generation studies of the nature of the GaAs(100) surface in air

We present in this paper a study of the non-linear optical response of the GaAs/air interface as measured by second harmonic generation. By careful choice of incidence and analysis electric vectors for the radiation the second harmonic response as a function of crystallographic orientation has been characterized for a GaAs(100) surface. The exact origin of the response remains unclear but comments on the surface specificity of the technique are made and the observed anisotropy in the data is explained in terms of the step density of the crystal surface.     

In situ atomic force microscopy studies of surface morphology,growth kinetics,defect structure and dissolution in macromolecular crystallization

Surface morphologies of thaumatin, catalase, lysozyme and xylanase crystals were investigated using in situ atomic force microscopy. For thaumatin, lysozyme and xylanase crystals, growth steps having a height equal to the unit cell parameter were produced both by screw dislocations and two-dimensional nuclei. Growth of catalase crystals proceeded in alternating patterns exclusively by two-dimensional nucleation and the successive deposition of distinctive growth layers, each having a step height equal to half the unit cell parameter. The shapes of islands on successive layers were related by 2-fold rotation axes along the 〈0 0 1〉 direction. Experiments revealed that step bunching on crystalline surfaces occurred either due to two- or three-dimensional nucleation on the terraces of vicinal slopes or as a result of uneven step generation by complex dislocation sources. Growth kinetics for thaumatin and catalase crystals were investigated over wide supersaturation ranges. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was seen. From the supersaturation dependencies of tangential step rates and the rates of two-dimensional nucleation, the kinetic coefficients of the steps and the surface free energy of the step edge were calculated. Adsorption of impurities which formed filaments on the surfaces of catalase and thaumatin crystals was recorded. Cessation of growth of xylanase and lysozyme crystals was also observed and appeared to be a consequence of the formation of dense impurity adsorption layers. Crystal growth resumed upon scarring of the impurity adsorption layer and clearing of the crystal surface with the AFM tip. Adsorption of three-dimensional clusters, which consequently developed into either properly aligned multilayer stacks or misaligned microcrystals was recorded. For catalase crystals, incorporation of misoriented microcrystals as large as 50×3×0.1 μm³ produced elastic deformations in growth layers of ≈0.6%, but did not result in the defect formation. Etching experiments on catalase crystals revealed high defect densities.     

Doping behavior of sulfur during growth of GaAs from the vapor phase

A study has been made of sulfur doping from H₂S in the vapor growth of GaAs using the Ga-AsCl₃-H₂ system. In order to separate the effects of the gaseous components the AsH₃-HC1-Ga-H₂ system was also investigated. A linear relationship was found to exist between the measured electron concentrations and PH₂S over a wide range. The $\text{P}{}^{\mathrm{<mtext>-1</mtext><mtext>2</mtext>}}{}_{\mathrm{<mtext>AS</mtext>}}{}_4$ and P^-2_GaCl (or: P^-2_HCl or P^-1_GaCl or P^-1_HCl) dependence of the rate of sulfur uptake combined with the pronounced increase in this uptake upon approaching the exact <100 > orientation suggests that the process is kinetically limited. Some model considerations are presented, based on the hypothesis that the sulphur is competing for adsorption on As sites with the reactive components in the system.     

On the kinetics of GaAs chemical transport and epitaxy in the system GaAs–I2

Kinetic analysis of experimental data on transport and epitaxial growth of GaAs with iodine in closed and in open tube systems has been carried out. The uniformly shrinking core model has been applied to study GaAs transport kinetics in the closed system. The kinetics of GaAs epitaxy in the open one is interpreted according to a model which assumes that in the initial stage of chemical reactions new phase nuclei are formed instantly at a constant number of active centers and that the nuclei grow isotropically.     

Isotopic method for studying the kinetics of crystal growth

An experimental method for studying the kinetics of crystal growth based on the knowledge of crystal size distribution in the crystallizer is presented. Residence time distribution was determined using the radioactive isotopes technique. The method was employed to determine the magnesium sulphate growth kinetics in high temperature conditions.     

The growth of InGaAsP by CBE for SCH quantum well lasers operating at 1.55 and 1.4 μm

InGaAsP has been grown by CBE at compositions of 1.1, 1.2 and 1.4 μm for the development of MQW-SCH lasers. The observed incorporation coefficients for TMI and TEG show strong temperature sensitivity while the phosphorus and arsenic incorporation behavior is constant over the substrate temperature range explored, 530 to 580°C setpoint. For higher substrate temperatures the growth rate increases with the largest growth rates occurring for the 1.4 μm quaternary. Low temperature photoluminescence indicates the possibility of compositional grading or clustering for the 1.1 μm material and also for the 1.2 μm material grown at the lowest substrate temperature. The final laser structure was grown with the InP cladding regions grown at 580°C with the inner cladding and active regions grown at 555°C. Using this approach we have successfully grown MQW-SCH lasers with the composition of the active In_xGa_1−xAs ranging from x=0.33 to x=0.73. Threshold current densities as low as 689 A/cm² have been measured for an 800 μm×90 μm broad area device with x=0.68.     

Auger electron spectroscopic studies of the water vapour adsorption on the (100) surface of austenitic chromium-nickel steel

The water vapour adsorption on the (100) surface of chromium-nickel steel was studied by Auger electron spectroscopy (AES) in the temperature range of 300 K to 700 K at water vapour pressures of 13.3 μPa to 133 mPa. The water vapour adsorption goes on more slowly than the oxygen adsorption under the same conditions. The measured kinetic isotherms may be described by logarithmic time laws as they are typical of the adsorption on inhomogeneous surfaces. With rising temperature of the samples the rate of adsorption as well as the quantity of fragments of the water molecule adsorbed during saturation state increases, i.e. the adsorption is activated. By means of the measured adsorption isotherms it may be concluded that a dissociative adsorption takes place.     

The kinetics of crystal growth of succinonitrile and cyclohexanol from the melt

The crystal growth kinetics of two organic substances with low melting entropies — succinonitrile and cyclohexanol — was studied in the wide range of supercoolings. The values of kinetic coefficients which connect growth velocity and interface supercooling are equal to 0.17 – 0.72 m/(s · K) for succinonitrile and (1.6 – 2.0) · 10⁻⁴ m/(s · K) for cyclohexanol. Viscosity of these substances was measured to compare the experimental data with theoretical dependences for continuous growth. It has been found that experimental kinetics of substances under study differs significantly from the expression of Wilson-Frenkel type.     

Dissolution kinetics of K-alum crystals as judged from the measurements of surface undersaturations

The normal dissolution rates, the slopes and the average velocities of dissolution steps of etch pits on the (111) face of a K-alum crystal, originating from dislocations with Burgers vector 110, were measured in relation to the surface undersaturations. From the mutual relations, it was shown that the dissolution was controlled by both diffusion and surface kinetic processes, although the contribution of the latter process was smaller than in the case of growth. It was also demonstrated that the normal dissolution rate was always larger than the growth rate. This is attributed to the fact that the slopes of growth hillocks are invariably smaller than those of etch pits.     

Growth and MBMS studies of reaction mechanisms for InxGa1−xAs CBE

The reaction mechanism involved in the growth of In_xGa_1−xAs lattice matched to InP by chemical beam epitaxy (CBE) was investigated using growth and modulated beam mass spectrometry studies. Emphasis was placed on elucidating how variations in substrate temperature, indium composition and arsenic overpressure influence growth kinetics and how sensitive changes in experimental conditions bring about deviations in the ideal stoichiometry (In_0.53Ga_0.47As) required for lattice matching to InP. Our observations indicate that the compositional variations in the InGaAs stoichiometry at high temperatures (> 485°C) arise because of the changes in the DEG decomposition: desorption branching ratio which is controlled by a temperature- and arsenic pressure-dependent surface population of indium atoms. The low temperature behaviour is governed by the availability of metal surface sites for triethylgallium decomposition which is increased by the presence of surface indium atoms.     

The estimation of crystallization kinetics from linear in n(L) versus L steady-state data including growth rate dispersion

A discussion of modelling of nucleation and crystal growth kinetics is presented for those mixedsuspension, mixed product removal (MSMPR) crystallizer data available in the literature, for which log population density data appears to be a linear function of crystal size. The modelling crystallization kinetics is done twice – once assuming that the system is obeying McCabe's Δ law, and again when the crystals exhibit growth rate dispersion (GRD). In the latter case the gamma, inverse gamma, log-normal, beta, Fisher, sum of two gammas, sum of two inverse gammas, sum of two log-normal, and sum of two constant rates of growth rate dispersion models are used for crystal growth rate estimation. Finally, McCabe's and GRD predictions are compared with the size-dependent growth rate estimations.